Devices capable of outputting a regulated DC voltage from an unregulated voltage are known for use in an integrated circuit. One example of a known circuit of this type is the "band gap core" circuit, or the "polarization circuit with a reference voltage output by a difference differential" as described in the "Analysis and Design of Analog Integrated Circuits" manual published by Paul T. GRAY and Robert G. MEYER in 1984, second edition under number ISBN 0471-81454 7, pages 4-181 and subsequent pages.
This type of circuit is capable of achieving a noise rejection ratio for noise from an unregulated power supply equal to about 40 to 70 decibels. This ratio is insufficient, particularly in the case of a power supply for a preamplifier for a signal output by the microphone in a camera with a universal standard bus port. In this type of camera, the main power supply has a large amount of noise due to the many sudden changes in power consumption. Typically, the regulated voltage is filtered, for example, using additional R.C. filters, but these filters are not very efficient at the low frequencies present in the audio range. Furthermore, these filters are not necessarily possible on integrated circuit chips.
Another disadvantage of self-polarized circuits like those described in the manual mentioned above, is that the output regulated voltage is necessarily less than the unregulated power supply voltage. Consequently, a sufficiently high unregulated voltage is necessary so that the regulated voltage after division is sufficient to be useable by standard circuits on the load side. This is not always the case, particularly on portable equipment such as cameras.
Accordingly, there is a need for a regulated power supply with a better noise rejection ratio for noise in the unregulated power supply voltage. This ratio is preferably between 100 dB or more. The better noise rejection ratio should also be at low audio frequencies, and the level of this power supply should be sufficiently high for use by these circuits on the load side.